1. Field of the Invention
The present invention relates in general to the art of electronic transistors and more specifically, to a novel material system and epitaxial structure for a modulation-doped field-effect transistor (MODFET) or lattice-matched and pseudomorphic high electron mobility transistors (HEMTs and pHEMTs).
2. Description of Related Art
A MODFET is a field-effect semiconductor transistor designed to allow electron flow to occur in an undoped channel layer so that the electron mobility is not limited by impurity scattering. MODFETs are used in a variety of electronic devices such as solid-state power amplifiers, low-noise amplifiers as well as satellite receivers and transmitters, advanced radar and fiber-optics operating in microwave, submillimeterwave and millimeterwave systems.
A conventional device includes a indium phosphide (InP) substrate; a buffer layer; a quantum well having a first quantum well barrier layer, a channel layer, a second quantum well barrier layer; a donor layer; and a barrier layer (also known as a Schottky layer). The two quantum well barrier layers are typically formed of a wide-bandgap semiconductor material such as aluminum antimonide (AlSb) or aluminum indium arsenide (AlInAs). The channel layer in the quantum well is formed of a narrow-bandgap semiconductor material such as gallium indium arsenide (GainAs) or indium arsenide (InAs). The donor layer and the barrier layer are typically formed of a wide-bandgap material such as AlInAs, AlSb or gallium antimonide (GaSb). The barrier layer may be doped to function as both the Schottky barrier layer as well as the donor layer, so that the epitaxial structure does not contain a separate donor layer.
A lattice-matched high electron mobility transistor (HEMT) is a type of MODFET, where a narrow-bandgap semiconductor material is lattice-matched to the wide-bandgap semiconductor material. A pseudomorphic high electron mobility transistor (pHEMT) is another type of MODFET where the narrow-bandgap semiconductor material is strained in relation to the wide-bandgap semiconductor material.
In these devices, a discontinuity in the energy gaps between the two wide-bandgap semiconductor epilayers layer and the narrow-bandgap semiconductor channel layer causes electrons to remain in the channel layer. Conduction of electrons therefore takes place in an undoped channel layer so that the electron mobility is not limited by impurity scattering.
For high-speed and low-noise microwave, submillimeterwave and millimeterwave applications, manufacturers have been concentrating on providing HEMTs having a gallium indium arsenide (Ga.sub.1-x In.sub.x As) channel with high In content because higher electron mobility may be achieved by increasing the In mole fraction. One type of state-of-the-art HEMT contains a binary channel wherein x=1 in the formula Ga.sub.1-x In.sub.x As. It is obvious that, with the high electron mobilities of InAs (values as high as 32,000 cm.sup.2 -s at room temperature have been demonstrated), a binary InAs channel would result in the fastest semiconductor device, provided that one has a suitable material to form the Schottky for the InAs channel. One possible candidate for incorporating the InAs channel is to use AlSb as the two epilayers. However, among other technical problems, AlSb is chemically unstable. It would therefore be desirable to find a chemically more inert material than AlSb as the barrier layer.
Another conventional high-speed, low-noise HEMT structure has a tertiary channel based on the AlInAs/GaInAs material system grown lattice-matched to the InP substrates. To improve the speed and low-noise operation of such a device, one can incorporate more In to the GaInAs channel (with a formula of Ga.sub.0.47 In.sub.0.53 As if lattice-matched to the InP substrate). It is also known in the art that incorporating a greater proportion of In in the Ga.sub.1-x In.sub.x As channel layer improves the performance of the device. However, the amount of In that can be added to the channel layer is limited because an increased proportion of In causes a lattice strain buildup in the channel layer. Although the strain buildup of the channel layer can be compensated, it is difficult to use AlInAs as a strain-compensating layer because increasing the amount of Al to shrink the lattice constant of this material increases the chemical reativity of the AlInAs and thereby makes the device unreliable. In addition, because AlSb is a binary material, it cannot be used to shrink the lattice constant.
In addition, it is desirable to manufacture a channel with a large sheet charge density in order to obtain a device with higher current-carrying capabilities. Increasing the conduction band discontinuity (.DELTA.Ec) between the donor layer and the channel layer increases the sheet charge concentration in the channel layer. Furthermore, a wide bandgap, large Schottky material is desirable in order to improve the breakdown and leakage characteristics of the device. Moreover, a high-resistivity, wide bandgap material will be needed in order to improve the turn-off characteristics of the device.